This invention relates to magnetic recording media, and more particularly to media obtained by coating uniaxially anisotropic, hexagonal, magnetic particles onto substrates substantially without orientation, which is suitable for use in high-density magnetic recordings.
For magnetic recording and reproduction purposes, magnetic particles of needle-like crystals such as .gamma.-Fe.sub.2 O.sub.3 or CrO.sub.2 particles have heretofore been oriented longitudinally in the planes of their respective coating layers so as to make use of their in-plane lengthwise remanent magnetization. However, the above recording and reproducing system has found to be accompanied by such drawbacks that stronger demagnetizing fields occur in magnetic recording media as their densities become higher and their recording and reproducing performance is poor particularly in the shortwave region. In order to overcome such strong demagnetizing fields and to carry out high-density recording, it is necessary to make the magnetic recording layer of each recording medium thinner while increasing its coercive force. Under the present circumstances, it is difficult to enhance the coercivity of magnetic recording layers beyond the present level. In addition, thinner magnetic recording layers will induce a new problem that the characteristics of their reproduction signals will be deteriorated. Accordingly, it is difficult to densify magnetic recordings beyond the present level as long as the above-mentioned conventional system, is used in other words, conventional needle-like magnetic particles oriented longitudinally in the planes of their respective coating layers and utilizing their in-plane lengthwise remanent magnetization.
It has also been proposed, with a view toward finding a breakthrough, to utilize the remanent magnetization in a direction vertical to the plane of each magnetic recording medium. In such a vertically-magnetized recording system, it is necessary to have an easy axis of magnetization in a direction vertical to the plane of each recording medium. Following recording media have been proposed.
As one example of such recording media, a film of a Co-Cr alloy is formed in accordance with the sputtering method on a surface of a base material. However, such a recording medium is not suitable for actual use, because it is accompanied by such drawbacks that both recording medium and magnetic head are subjected to considerable wear through their sliding contact, the recording medium is hard to handle due to its poor flexibility, and suffers from its poor productivity.
As another example, a so-called coating-type vertically magnetized recording medium has also been proposed. It is generally fabricated by coating on a substrate magnetic particles together with an organic binder to form a magnetic recording layer. It has been proposed to employ, as magnetic particles, powder of Fe.sub.3 O.sub.4 polyhedron or barium ferrite which may optionally contain further substituent element or elements and orient such magnetic particles in directions vertical to the plane of each recording medium. Recording media of this type have improved recording and reproduction performance in the shortwave region and enable a densification of the magnetic recordings. However, it involves such shortcomings that it can achieve only poor contact with a magnetic head due to an excessively high degree of roughness on the surface of its magnetic recording layer and its reproduction output is thus unstable compared with the conventional recording media having needle-like magnetic particles oriented longitudinally in their magnetic recording layers. It is also accompanied with another demerit that no satisfactory recording is feasible by conventional ring-shaped magnetic heads.
From the standpoint of fabrication technology, all the conventional coating-type magnetic recording media have been subjected to an orientation treatment in a magnetic field. Irrespective of using an in-plane lengthwise remanent magnetization or a vertical remanent magnetization, it has been indispensable to subject each magnetic recording medium to an orientation treatment in a magnetic field so as to orient the easy axis of magnetization of each magnetic particle in a desired direction. Such an orientation treatment in a magnetic field requires special facilities such as, for example, orientation magnets and mechanical orientation apparatus. It will certainly be advantageous if such an orientation treatment could be obviated, because it will lead to a simpler fabrication process, energy and man power saving, and a lowered fabrication cost.
It has thus been attempted to use magnetic recording media which were obtained by coating a resinous coating composition containing needle-like magnetic particles onto surfaces of substrates and then simply drying them without applying any orientation treatment in a magnetic field. Here again, it is up to the way of utilization of remanent magnetization of each magnetic particle oriented in a direction vertical to the plane of each recording medium whether high-density magnetic recording is feasible or not. However, the extent of high densification or recording attained was in fact far below the intended level. This is attributed to a very small remanent magnetization component in the vertical direction. In view of these experiences, an application of the orientation treatment in a magnetic field has been considered to be an indispensable step for the fabrication of magnetic recording media.
As has heretofore been described in detail, it was hardly possible to achieve the high densification of recordings by means of needle-like magnetic particles. Fe.sub.3 O.sub.4 polyhedrons and substituted barium ferrites were capable of attaining high-density recordings. However, they were not satisfactory for actual use because they involved various shortcomings and problems as recording media. In view of the history of development of magnetic recording media, the orientation treatment in a magnetic field has been believed to be an indispensable step and it was certainly out of the question to seek an advantage from their fabrication viewpoint by eliminating the orientation treatment in a magnetic field.